Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft drives the generator rotor either directly (“directly driven”) or through the use of a gearbox. The operation of the generator produces the electricity to be supplied into the electrical grid.
Wind turbines are often grouped together in so-called wind farms. In a wind farm there may be a relatively short distance between wind turbines as a trade-off is made between available land and wind farm power production, i.e. number of wind turbines. The interaction of the wind with a first upstream wind turbine produces a wake including a blade tip vortex with high local air velocities. Such a wake may be projected onto a downstream second wind turbine located nearby. When the wake shed from a first wind turbine intersects with the swept area of a second wind turbine located downstream, it may cause high loads (particularly vibrations) in this wind turbine in case specific load mitigation strategies have been implemented. This phenomenon may cause a reduction of electrical power production. These unwanted effects are mainly induced by the high air speed that accompanies the vortexes generated, especially at the blade tips of the first wind turbine. High loads may increase fatigue. It is important to note that the definition of a wind turbine as an upstream wind turbine or a downstream wind turbine does not depend on the wind turbine itself but on the relative position of the wind turbines and on the wind direction.
Currently known wake management strategies are generally based on stopping either the first or the second wind turbine or on power curtailment of at least one of the first wind turbine and the second wind turbine. This strategy is activated when a predetermined wind direction, capable of generating wake effects, is sensed. Said predetermined directions are commonly defined beforehand on the basis of simulation analyses, which take into account both statistical wind data and arrangement of the wind turbines. The new operation regime is usually maintained until the wind direction changes to a safer situation in which there is no such potential wake situation. This strategy can negatively affect the total electrical power production of the wind farm as wind turbine power production is significantly reduced, if not completely eliminated, upon detection of certain wind conditions. Furthermore, said commonly used management strategies do not typically comprise any optimization to account for other specific conditions e.g. when the wind turbine potentially receiving the wake situation is already not operating.
Furthermore, increasing the distance between the wind turbines, such that a wake shed from a wind turbine located upstream under no circumstances intersects with the swept area of a wind turbine located downstream, may also be used as an additional method to mitigate wake effects. Nevertheless, this may not be possible in certain sites with specific constraints, as this strategy would require a larger area for installing the wind farm. Moreover, the electrical infrastructure, and thus the costs, would also be increased.
Other known wake management strategies are based on displacing the rotor plane either by yawing or tilting the wind turbines located upstream after detecting changes in the wind direction. Document WO2004011799, for example, discloses such strategies. However, systems for tilting a rotor are generally not provided on wind turbines and their implementation could be very costly. On the other hand, activation of the yaw system, which is indeed provided on wind turbines for alignment to varying wind directions, involves moving the entire nacelle, and rotor with blades, which correspond to a big mass and inertia. Significant power is needed to displace such a big mass, said power thus reducing overall efficiency of the wind turbine by reducing electrical energy fed into the grid. Besides, the use of the yaw system may be rather slow and, again, quite expensive.
Thus, there still exists a need to provide a method of operating a wind farm that at least partially reduces some of the aforementioned problems.